1. Field of the Invention
The present invention relates to a surface measurement system and more particularly to an optical three-dimensional (3D) imaging system and method for monitoring a surface roughening process in light emitting diode (LED) chip manufacturing.
2. Related Art
In recent years, high brightness LEDs (HBLEDs) have found increasing uses in automotive, traffic, and industrial lighting as well as in display applications. One key performance parameter of a HBLED is its overall efficiency. This parameter is determined by two factors: 1) the internal efficiency of its active layer in converting electric energy to light when the LED is energized and 2) the light extraction efficiency, i.e. the fraction of light that is able to escape the chip through all exposed surfaces. Because the internal efficiency has been improved to approach the theoretical limit, emphasis now has been placed on improving the light extraction efficiency. One approach to improve the light extraction efficiency roughens a surface of the HBLED during manufacturing. The roughening can be done using photo-electrochemical (PEC) oxidation and etching of an n-gallium nitride (n-GaN) layer, which is described, for example, in U.S. Publication 2009/0315055, entitled “Photoelectrochemical Roughening Of P-Side-Up GaN-Based Light Emitting Diodes”, and filed by Adele Tamboli et al.
For a HBLED manufacturer, monitoring this PEC surface roughening process is very important. Currently, there are several types of systems that can measure PEC roughened LED surfaces. One type of system, which is described in U.S. Pat. No. 7,563,625, entitled “Method Of Making Light-Emitting Diodes (LEDs) With Improved Light Extraction By Roughening”, and issued to Chuong Anh Tran et. al. on Jul. 21, 2009, uses a scanning electron microscope (SEM) to measure such roughness. Although the SEM provides high resolution images of a surface, it is difficult to obtain quantitative vertical dimensions unless a sample is cut and a cross-sectional measurement is performed. Because the SEM is generally a destructive method, it is undesirable in a commercial manufacturing environment. In addition, even if not destructive, the SEM has to operate in a vacuum environment, thereby resulting in relatively low throughput.
Another type of system, which is described in U.S. Publication 2007/0114511, entitled “III-Nitride Compound Semiconductor Light Emitting Device”, and filed by Chang-Tae Kim et. al., uses an atomic force microscope (AFM) to monitor the surface roughening process. Although the AFM is capable of generating very high resolution images, it is difficult to obtain repeatable surface roughness results. Moreover, the AFM result is influenced by the condition of its tip, which needs to be replaced frequently to ensure accuracy. Yet further, the AFM requires highly trained operators, and is highly susceptible to environmental noise and vibrations. Finally, the AFM semi-contact method is somewhat destructive. Therefore, the use of the AFM is also not a commercially viable method for in-process surface roughness monitoring.
Yet another type of system, which is described in U.S. Pat. No. 4,511,800, entitled “Optical Reflectance Method For Determining The Surface Roughness Of Materials In Semiconductor Processing”, and issued to Gunther Harbeke et. al. on Apr. 16, 1985, and U.S. Pat. No. 6,452,678, entitled “Reflectance Method For Evaluating The Surface Characteristics Of Opaque Materials”, and issued to Randhir Thakur et. al. on Sep. 17, 2002, uses optical reflectance-based methods instead of high resolution imaging to monitor surface roughness (e.g. for silicon wafers). Although these optical reflectance-based methods are fast and non-destructive, they do not quantify the raw reflectance data into useful information about a PEC roughened LED surface, nor do they adequately address the need for LED production process monitoring.
Therefore, a need arises for a non-destructive method that is fast, easy to use, and relatively inexpensive to implement to monitor PEC surface roughening in HBLED manufacturing.